<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .btn {
        width: 100px;
        height: 40px;
        background-color: pink;
        font-size: 20px;
        text-align: center;
        color: #fff;
        line-height: 40px;
    }
</style>

<body>
    <button class="btn">开关灯</button>
    <script>
        var btn = document.querySelector('.btn');
        var flag = 0;
        btn.onclick = function() {
            if (flag == 0) {
                document.body.style.backgroundColor = 'black';
                flag = 1;
            } else {
                document.body.style.backgroundColor = '#fff';
                flag = 0;
            }

        }
    </script>
</body>

</html>